When printing, for example, using a color inkjet printer, the color is applied at once to the paper. Each color generally ends up where it is expected. When printing, in commercial printing presses, a document consisting of more than one color of ink requires that the single page pass through the printing press multiple times as each color is applied to the paper. Sometimes the paper or the plates applying the ink shift. It might be a tiny shift but sufficient to be noticeable. For example, a white gap may appear between a cyan letter that is supposed to be touching a magenta box. When this happens, the color is said to be out-of-register. Color-to-color misregistration refers to misregistration between color separations in a printed image. Misregistration in printed halftoned images can result in image defects such as, for instance, white gaps between a two color edge.
One general approach for correcting for misregistration is to expand one of the abutting regions' separations to fill the misregistration border region with a color determined to minimize the visual effect when printed. Borders or edges that have been expanded from a region of one color to a region of another color, in this manner, are said to be “spread”, i.e., the submissive color is spread into an area of the dominant color. A border which has been expanded is referred to as a “trap”. The zone within which color has been added is called the “trap zone”. Misregistration of colors in a printing device can, in many instances, be compensated for by a process often referred to as “trapping” which is a term of art used to describe a hardware and/or software process for compensating for a small amount that the paper tends to wander as it travels through the print system. Trapping compensates for mechanical shifts or stretching of paper or plates in the printing process and provides an overlap of colors to prevent unprinted paper from showing in the final printed product. This is not to be confused with the terms “wet trapping” or “dry trapping” which are terms used to describe behaviors between inks or between ink and paper. Trapping helps preserve the integrity or the shape of an image such as block serifs. Without trapping, unsightly gaps may be visible between two colors that are supposed to be touching. With trapping, one color is made to overlap the other by extending that color into a surrounding area. This allows the colors to keep touching one another, even as the paper wanders.
Trapping is often accomplished with features built into software algorithms devoted to trapping methods. Many commercial printing devices perform a trapping method. In many systems, applications for color trapping require edge detection be performed and an orientation direction of that edge determined. U.S. patent application Ser. No. 12/754,096, entitled: “Color Trapping On A Halftoned Bi-Level Bitmap”, by Meng Yao, (U.S. Publication No. 2011/0243429), discloses a binary color detection algorithm which checks the orientation of edges of different color planes at a pixel locations in the bi-level bitmap. While this algorithm produces robust results, edge orientation determination is complicated and expensive to implement.
Accordingly, what is needed in this art is a system and method for determining an orientation direction of a color edge at a given pixel location in a halftoned color image which provides a cost-effective solution that can be readily implemented in hardware to enable the integration of sophisticated image processing techniques into low cost print devices.